Two-stroke engines with one or more cylinders generally include, associated with each of the cylinders, a crankcase, called a pump crankcase, communicating with one end of the combustion chamber of the cylinder and providing introduction of a fresh gas into the cylinder, by at least one line and a transfer opening. The piston, which moves in a reciprocating motion in the cylinder, also ensures a suction and a compression of the fresh gases in the pump crankcase. An intake valve, arranged on the pump crankcase, allows introduction of the fresh gases into the crankcase when the piston moves in the opposite direction of the crankcase, with these fresh gases being thereafter compressed and ensuring a closing of the intake valve, when the piston moves in the direction of the crankcase. When the corresponding openings of the cylinder are uncovered by the piston, fresh gases are introduced into the cylinder through the lines and the transfer openings and they generate a fresh gas scavenging intended to replace the waste gases which are discharged through exhaust openings generally offset with respect to the transfer openings. The piston moves away from the crankcase so as to compress the gases contained in the cylinder. The ignition and the combustion of the mixture of air and fuel thereafter generate the motive displacement of the piston towards the crankcase.
Two stroke engines with one or more cylinders are particularly known which comprise, associated with each of the cylinders, a device for a pneumatic injection of fuel providing spraying of a liquid fuel by a gas under pressure which is generally compressed air, and an introduction of the carburetted mixture obtained into the engine cylinder. Feeding and scavenging of the cylinder with fresh air is then achieved independently of the pneumatic injection of the fuel which is triggered at a predetermined time of the working cycle of the engine.
More particularly, the fuel pneumatic injection device may include a chamber which may be communicated with the inner chamber of the cylinder by a specific device, hereinafter referred to as a pneumatic injection control device. The pneumatic injection control device for each cylinder may, for example, include an automatic valve, a controlled valve, or a rotary plug.
In this type of pneumatic injection, the gas under pressure may be supplied from either an auxiliary tank or a part of the engine producing gas under pressure.
It has been proposed to supply compressed air to the pneumatic injection control device from a capacity of accumulator, which capacity or accumulator is reloaded by the pump crankcase so as to ensure a fresh air feed and scavenging of the engine cylinder.
Engines having characteristics outlined hereinabove are proposed in, for example, FR-2,623,854 and FR-2,656,636.
The subject matter of the present invention is particularly applied to the engine type described above and relates to the first order balancing of reciprocating or vibrating masses of the engine.
FR-2,558,232 proposes an internal combustion engine in which the first order balancing of the forces generated by the reciprocating or vibrating masses of the engine is achieved by additional masses connected in rotation to the cam shaft driven in rotation by the crankcase and by additional masses connected in rotation to the crankcase.
Those skilled in the art are aware that the first order balancing of an engine requires a shaft, other than the crankcase, to be rotated at the same speed in the opposite direction of the crankcase.